The present invention relates to monolithic multilayer capacitors and, in particular, to a method of making end terminations for a monolithic multilayer capacitor.
A monolithic multilayer capacitor includes a body portion, referred to herein as a "chip," and also includes a pair of end terminations. Such a capacitor has a stacked configuration in contrast to a wound configuration. In the stacked configuration, there are alternating conductive and dielectric layers, with the conductive layers defining first and second sets. These layers are contained in the chip. In the capacitor, all the conductive layers in the first set are electrically connected together at one end of the chip by one of the pair of end terminations; all the conductive layers in the second set are electrically connected together at the opposite end of the chip by the other of the pair of end terminations.
Various types of materials can be used to make a monolithic multilayer capacitor. When ceramic is used as the dielectric, the capacitor is referred to as a monolithic ceramic (MLC) capacitor. In an MLC capacitor, a platinum/palladium alloy is commonly used for the conductive layers. In other monolithic multilayer capacitors, any of various materials, including silver, gold, and platinum, is used for the conductive layers. In any case, the capacitor is functional only if at each of the opposite ends of the chip the respective end termination provides a good electrical connection to the edges of the conductive layers in the chip, such that all the conductive layers in one set cooperate as one electrode and all the conductive layers in the other set cooperate as another electrode.
Monolithic multilayer capacitors have many uses and have substantial commercial importance. A substantial amount of research, development, and design effort has been devoted to methods for manufacturing such capacitors, particularly to methods for providing the required end terminations.
One prior art approach that has been used to electrically connect such conductive layers together involves a liquid coating process. A representative example of this liquid coating process can be more particularly described in the concrete context of the manufacture of MLC capacitors. In this context, the prior art liquid coating process entails preparing an ink comprising silver and a glass frit for use as a coating material. In sequence, one end of the MLC chip is dipped into the ink, and later the opposite end is dipped into the ink. After dipping, the chip is placed in an oven and subjected to a firing cycle where the glass frit bonds to the ceramic and the silver in the ink mixture provides the electrical continuity and electrical termination joining the conductive layers within the chip and thus provides the end terminations for the resulting functional capacitor. A capacitor prepared in this manner can be used as a surface-mounted device; that is, the capacitor can be bonded to a printed circuit board by directly reflow soldering or by using conductive epoxy that electrically connects plated conductors on the board directly to the end terminations of the capacitor. Alternatively, a separate lead can be attached to the film at each end of the capacitor to provide a pair of leads for inserting into plated holes in a circuit board or for otherwise connecting the capacitor to the other electrical circuitry with which it is to be used.
For a surface-mounted capacitor, it is important that each end termination have a configuration such that the end termination not only covers the end of the chip but also forms a thin, surrounding band adjacent the end. This is important for two reasons. First, during surface mounting of the capacitor, the solder will "float" the capacitor slightly above the board, thereby providing clearance beneath the bottom surface of the capacitor and allowing use of cleaning fluids to remove residual flux. Second, the capacitor should be symmetrical so that it does not have to be specially oriented during the assembly operation preceding the soldering operation. If, instead of a surrounding band, the end termination had a strip portion covering only one surface, the capacitor would be asymmetrical and would have to be specially oriented during the assembly operation.
Various problems inhere in the foregoing prior art process. One of these problems relates to controlling the width of the band of termination material around the chip and, in addition, controlling the reaction and interaction between the materials which comprise the glass frit and the ceramic material that is used in the MLC capacitor. To provide a desired width for the band of termination material around the sides of the chip adjacent the end, it is necessary to control not only the actual depth of dipping and but also the rheology of the ink. In addition, it has been found that the rheology of the silver and glass frit ink changes with time and introduces another variable into the process.
Another problem arises from a particular phenomenon encountered in the firing cycle step of the process; that is, the glass in the ink tends to diffuse and come to the surface. When the glass comes to the surface, the glass severely hinders the solderability of the end termination material and likewise the platability of other metals on the end termination material.
An inadequate control of the reaction and interaction between the frit and the ceramic can cause further problems in both surface-mounted capacitors and capacitors that have leads attached to the end terminations. If the relationship between the ceramic and the frit is not properly controlled, the step of surface mounting or of attaching leads is frequently found to cause microfractures in the end termination film material which causes yet bigger fractures, failures and malfunctions.
Another problem relates to the expense involved in production operations such as the sequential ink-coating steps in which one end at a time of the MLC chip is dipped into the ink. These steps are not only time consuming but also difficult to carry out with miniature capacitors. The dimensions of such a miniature capacitor can be about 0.080" long, by 0.050" wide, by 0.050" thick. Another factor that adds to the cost of the capacitor is the cost of the noble metal, such as silver, that is used in the coating ink.
In addition to the above-described process, the prior art includes U.S. Pat. No. 3,992,761. This patent discloses an approach to the problem in a production operation of providing an individual termination at each end of many multilayer capacitor chips. The disclosed approach is to mount the capacitor bodies in a support sheet and encapsulate the support sheet and the capacitor bodies in a plastic block with the ends of the capacitor body being exposed at opposed surfaces of the block. According to the disclosed approach, the exposed ends of the capacitor bodies are simultaneously plated via an electroless plating process with a termination film and then with a solder film and the terminated capacitor bodies are thereafter removed from the block.
According to the disclosed approach, the way to extract the capacitors from the block is to place the block in a solvent bath until all of the plastic is dissolved to separate the individual capacitors from the block, and then, after the plastic has been completely dissolved, remove the capacitors from the solvent and then wash them to remove the solvent.
It is apparent that numerous disadvantages are involved in this disclosed approach. One such disadvantage is the introduction of additional assembly steps. These additional assembly steps include attaching the MLC chips to a support sheet and thereafter encapsulating the sheet and mounted chips in an encapsulating block and controlling the dimensions of the block so that the ends of the chips extend from opposite ends of the encapsulating block. Further, after the plating operations, the encapsulating body thereafter has to be removed, and the capacitors detached from the mounting sheet. Using such additional assembly and disassembly steps would inevitably entail associated expenses; further, using the disclosed approach entails electroless plating operations and associated material costs of the baths. Another disadvantage that appears is that the use of the solvent would have an adverse effect on the electrical characteristics of the end termination film and the capacitor.
Another prior art patent directed to a process for making end terminations is U.S. Pat. No. 4,613,518. Like the approach disclosed in U.S. Pat. No. 3,992,761, the process for making end terminations in accord with the disclosure of U.S. Pat. No. 4,613,518 involves a step of providing a partial body cover, in this case a thin coating of epoxy, before carrying out the step of forming the end terminations, in this case by vapor deposition of nickel.
As indicated by the foregoing background, there has existed a need for an improved process for making end terminations for a monolithic multilayer capacitor.